The scale on a map is 5cm : 3km. If two cities are 20cm apart on the map, what is the actual distance in km between the two cities?
The scale means that every 5cm on the map corresponds to an actual distance of 3km. A distance of 20cm on the map is the same as 4 $\cdot$ 5cm. The actual distance between the two cities is 4 $\cdot$ 3km, or 12km.